An NLO material is a material which has optical properties that are modified by light as it passes through the material. The modification of the optical properties may be caused by an induced electronic charge displacement (polarization) that acts as an oscillating dipole. The oscillating dipole may cause the material to emit a photon. When the polarization of the material is linear, the emitted photon has the same frequency as the light incident upon the material. If the polarization is nonlinear, the frequency of the light emerging from the material may be some integer value times the frequency of the incident light. For instance, the net effect of frequency doubling is that two photons with a frequency .OMEGA. combine to generate a single photon having a frequency equal to 2.OMEGA.. Thus, propagation of the waves in synchronization (phase-matching) allows the light's frequency to double. Franken first discovered that NLO materials were capable of such second harmonic generation (SHG) in 1961. Ann. Rev. Mater. Sci., 16:203-43 (1986). Phase-matching ability is a critical aspect of all NLO SHG materials.
NLO materials may also exhibit photorefractive phenomena. A photorefractive material is one in which low intensity light (light having an intensity of about 1/1000 of a watt) semi-permanently alters the material's refractive index (defined as the speed of light in a vacuum relative to the speed of light through the material). Photorefractive materials generally have easily distorted crystal lattices and virtually all such materials contain defects in the crystal structure that act as charge carriers. On an atomic level, the electrons in a photorefractive crystal are displaced by the oscillating electric field of an incident light wave. The ability of the electrons to be displaced is governed by:
(1) the direction and magnitude of the electric field; PA1 (2) the nature of the potential well of the electron; and PA1 (3) the frequency of the applied field.
One purpose of NLO materials is to introduce strong coupling between the electro-magnetic field of a primary light wave and a secondary light wave within the crystalline lattice. This coupling is referred to as internal modulation. Alternatively, the electro-magnetic field of a primary light wave may be modulated by coupling it to an externally applied electric field.
The light emerging from an internally or externally modulated light wave can be used for a number of practical applications, including: optical interconnection of electronic circuits; laser surgery; hologram generation; electro-optic wave guides utilizing surface-applied dopents; and for high-speed, light exploiting computers.
A number of NLO materials are known and such materials have been used in a variety of devices. For instance, LiNbO.sub.3 was found to be photorefractive nearly 25 years ago. Scientific American, 62-74, October, 1990. Also, Chemistry of Materials, Vol. 1, No. 5, 492-509 lists a number of NLO compounds, most of which are derivatives of potassium titanyl phosphate. Other commonly used NLO materials include KDP and urea.
NLO materials have also been described in previous U.S. patents. For instance, Chuangtian et al.'s U.S. Pat. No. 4,826,283 describes an NLO device made from single crystals of LiB.sub.3 O.sub.5. Huignard et al.'s U.S. Pat. No. 4,659,223 provides an interferometric device for measuring angular rotational speed. The device employs an amplifying photorefractive crystal, such as a bismuth-silicon oxide crystal or a barium titanate crystal.
Although a number of NLO compounds are known, most of these materials exhibit deficiencies that limit their utility, including: excessive energy requirements to induce the NLO effect; minimal SHG efficiency; NLO crystal damage resulting from exposure to high powered lasers; excessive absorption and light-scattering of incident light; and excessive production costs and synthesis times. Chemistry of Materials, Vol. 1, No. 5, 492-508 (1989). For instance, KTP (KTiOPO.sub.4) costs approximately $300,000 per ounce and the Airtron method of synthesizing KTP takes approximately five to six weeks to complete.
Thus, there is a need for NLO compounds that overcome these problems and are suitable for use in high-speed optical computers and other such devices.